In vehicles provided with a gasoline engine, it has been customary to determine the optimum engine speed for throttle position in view of improvement of fuel consumption and/or purification of exhaust gas. With a continuously variable transmission it is possible to run an engine at a desired speed regardless of the vehicle speed. To this end, it has been customary to control a continuously variable transmission by a speed-ratio controller so as to realize the above-mentioned desirable correspondence between throttle position and engine speed.
In particular, the conventional standard hydrostatic continuously variable transmission includes a variable speed hydraulic transmitting system between a pair of hydraulic pump/motors at least one of which at the input side is of a variable displacement type, and is so designed that the speed ratio of the output speed to the input speed can be rendered nearly zero with the displacement of the input hydraulic pump/motor acting as a pump being kept zero.
The conventional hydro-mechanical continuously variable transmission comprises: a differential mechanism having a first, a second and a third input/output terminal, a low-speed mechanical transmitting system between the first and second input/output terminals, and a high-speed mechanical transmitting system between the first and third input/output terminals; a hydraulic transmitting mechanism comprising a first hydraulic pump/motor of a variable displacement type the input/output shaft of which is connected to the second input/output terminal of the differential mechanism and a second hydraulic pump/motor of a variable displacement type the input/output shaft of which is connected to the third input/output terminal thereof thereby to form a variable speed hydraulic transmitting system; a low-speed clutch for connecting and disconnecting the transmitting terminal of the above-mentioned low-speed mechanical transmitting system to and from a rotary element provided at the output side; and a high-speed clutch for connecting and disconnecting the transmitting terminal of the above-mentioned high-speed mechanical transmitting system to and from another rotary element provided at the output side, whereby either a low-speed or a high-speed mode can be selected by operating either one of the above-mentioned clutches; the first input/output terminal being connected to an engine and the rotary elements, to an axle of the vehicle.
The speed-ratio controller that is composed chiefly of an ECU (electronic control unit) detects throttle position or accelerator pedal position and engine speed and controls the displacement of the hydraulic pump/motor so that the engine speed is kept at a desired value in accordance with the throttle position.
The above control will be described in detail. As the driver steps on the accelerator pedal, the throttle valve is opened, and the desired engine speed corresponding to the throttle position is obtained from a chart of throttle position/desired engine speed, and the displacement of the hydraulic pump/motor, that is, the speed ratio of the continuously variable transmission is controlled so as to keep the actual engine speed at the desired value.
The vehicle provided with the conventional continuously variable transmission has the following three problems:
First, a time delay exists between the time the accelerator pedal is stepped on to start the vehicle and the time the vehicle begins to start, with resulting deterioration of response. In particular, if the throttle position is subjected to a stepwise increase as the driver steps on the accelerator pedal, the desired engine speed SD greatly increases stepwise. The actual engine speed SE, however, increases with the first order lag accompanied with loss time, so that for some time the actual engine speed SE remains lower than the desired engine speed SD. It is only after the actual engine speed SE exceeds the desired engine speed SD that the engine begins to be loaded by increasing the displacement of the hydraulic pump/motor acting as a pump which has until then been zero so as to lower the actual engine speed SE. Consequently, hydraulic fluid begins to flow into the hydraulic pump/motor at the output side which acts as a motor, so that the vehicle starts moving. The above arrangement has a disadvantage that when the accelerator pedal has been stepped on, until the actual engine speed SE becomes high, the displacement of the hydraulic pump/motor acting as a pump remains unchanged and no hydraulic fluid is supplied to the hydraulic pump/motor acting as a motor, so that the vehicle does not move.
A second disadvantage is that there occurs a great overshooting of the actual engine speed SE. Even when the actual engine speed SE exceeds the desired engine speed SD and the speed-ratio controller begins to increase the displacement of the hydraulic pump/motor acting as a pump, the actual engine speed SE is not suppressed until the engine load torque due to the increasing displacement increases to exceed the engine output torque corresponding to the throttle position at that time, so that the actual engine speed SE continues to increase to cause an overshooting. For example, when the accelerator pedal is stepped on comparatively deeply as is usual with ordinary drivers, the throttle valve is greatly opened stepwise so that the engine output torque corresponding to the throttle position increases greatly. At that time, however, since the displacement of the hydraulic pump/motor acting as a pump remains small, the engine load torque remains small even when the high pressure side of the hydraulic transmitting system has reached a relief pressure, so that the actual engine speed SE rises over the desired engine speed SD so as to cause the relief valve to operate. In other words, while the vehicle speed remains low, the engine output torque is excessively great, so that with the relief valve open to cast away excessive energy, the engine is running almost under racing condition to cause an overshooting of the actual engine speed SE. As a result, it becomes impossible at least during that period of time to run the vehicle with low fuel consumption.
The third disadvantage is that when the vehicle is started, the pressure fluctuates greatly with resulting great fluctuation of the torque. After the actual engine speed SE has overshot in the above-mentioned manner, the speed-ratio controller rapidly increases the displacement of the hydraulic pump/motor acting as a pump in order to recover the delay in control, so that the flow of hydraulic fluid through the hydraulic transmitting system suddenly increases to cause the pressure and the torque to increase rapidly. This causes acceleration at the time of starting the vehicle to become excessive thereby to prevent the vehicle from starting smoothly and give the driver a feeling of rushing out.
The invention has been accomplished in view of the above-mentioned problems, and its object is to solve the above-mentioned first and third problems concerning the starting characteristic which is of primary importance in vehicles thereby to ensure stable, smooth driving and speed changing at the time of starting the vehicle, and to solve the above-mentioned second problem thereby to improve fuel consumption.